Anodizing process, with low environmental impact, for a woodpiece of aluminum or aluminum alloys

ABSTRACT

The process for anodizing a workpiece of aluminum or aluminum alloys provides for the use of the workpiece as the anode of an electrolytic cell in the presence of an aqueous acid solution which contains from 10 to 200 g sulfuric acid and from 5 to 200 g L(+)-tartaric acid.

DESCRIPTION

[0001] The present invention relates to an anodizing process for aworkpiece (such as, for example, a part or a component of a system) ofaluminium or aluminium alloys, this definition being intended tocomprise pure or almost pure aluminium, as well as combinations thereofin all percentages with other elements.

[0002] In greater detail, this process makes use of the above-mentionedworkpiece as the anode of an electrolytic cell in the presence of anaqueous acid solution in order to form on the surface of the workpiece acoating of aluminium oxide.

[0003] This coating results in better properties concerning theresistance to corrosion and the adhesion of paints, bonding agents andother organic coatings suitable for improving the appearance or theresistance to corrosion of the finished workpiece.

[0004] Conventionally, this acid solution substantially comprisessulphuric acid at high concentrations or chromic acid, the latter beingthe compound mainly used in the field of aeronautics.

[0005] The problem addressed by the present invention is to provide ananodizing process having a lesser toxicological impact on theenvironment in comparison with the known types, avoiding the use ofchromic acid whilst nevertheless obtaining anodized workpieces havingproperties which are at least equivalent to those of workpieces obtainedby conventional means.

[0006] This problem is solved according to the invention by an anodizingprocess of the type indicated in the introduction to the presentdescription and characterized in that the aqueous acid solution containsfrom 10 to 200 g sulphuric acid and from 5 to 200 g L(+)-tartaric acidper liter of solution.

[0007] The process according to the invention has the advantage ofeliminating, or in any case greatly reducing, the production ofindustrial waste, in particular waste containing compounds of chromium,without causing the formation of other waste materials having particulartoxic or dangerous properties.

[0008] The process according to the invention further has the advantageof having a cycle time which is approximately 40% short of that of theanalogous process which makes use of chromic acid, producing anodizedworkpieces having properties at least equivalent if not superior.Consequently, the process according to the invention allows aconsiderable reduction in running costs to be obtained in that itresults in lower costs for the treatment of the waste and involves asubstantially lower rate of consumption for the different chemicals usedfor replenishing the baths, which have an operational life greater thanthose used in known processes.

[0009] The aqueous acid solution of the process according to theinvention preferably contains from 20 to 80 g sulphuric acid and from 30to 120 g L(+)-tartaric acid per liter of solution.

[0010] Again preferably, the aqueous solution is maintained, during theanodizing treatment, at a temperature of between ambient temperature and120° C. and, more preferably, between 25 and 35° C.

[0011] The electrolytic cell, in which the process according to theinvention is carried out, is preferably subjected to a voltage ofbetween 1 and 120 V and, more preferably, of between 10 and 30 V.

[0012] The process according to the invention advantageously has aduration of between 5 and 120 minutes and, preferably, of between 5 and30 minutes.

[0013] The anodizing process is generally preceded by a cleaning and/ordeoxidizing treatment of the workpiece to be anodized and can befollowed by a sealing treatment and, optionally, rinsing for theanodized workpiece. In principle, these treatments are of conventionaltype and completely compatible with the present invention and can beselected at will from treatments known to this end by the person skilledin the art.

[0014] The workpiece advantageously has, upon completion of theanodizing treatment, an oxidized surface coating having a thickness ofbetween a few micrometers and several tens of micrometers.

[0015] There now follow, merely by way of non-limiting, illustrativeexample, examples of the anodizing process according to the invention,as well as a comparative example performed by conventional techniques.

EXAMPLE 1

[0016] Anodizing a 2000-Series Aluminium Alloy with Sulphuric-TartaricAcid

[0017] A workpiece (of a 2000-series aluminium alloy having dimensionsof 150×100×1 mm) was first subjected in a conventional manner tocleaning and deoxidizing treatments, such as de- greasing by immersionfor approximately 10 minutes, rinsing by immersion in drinking water forapproximately 3 minutes, pickling/deoxidation by immersion forapproximately 10 minutes and rinsing by immersion in drinking water,preferably followed by further rinsing by immersion in deionized waterfor a total of approximately 3 minutes.

[0018] The workpiece was then used as the anode of an electrochemicalcell, in which the cathode was constituted by corrosion-resistant steelof the type AISI 321 and having a surface area equal to or greater thanthat to be oxidized and in the presence of an aqueous acid solutioncontaining 45 g sulphuric acid and 80 g L(+)-tartaric acid per liter ofsolution and was maintained at a temperature of between 30 and 35° C.The workpiece was kept completely immersed in the acid solution.

[0019] The electrical voltage was increased from an initial value of 0 Vto a value of 18 V at a rate in the order of 3.6 V/min. The finalvoltage value was maintained for 25 minutes, producing on the workpiece,upon completion of the anodizing treatment, an oxidized coating having athickness of approximately 5 μm.

[0020] The anodized workpiece was then subjected to treatments which areconventional per se, such as rinsing with drinking water, preferablyfollowed by further rinsing with deionized water for an overallimmersion time of approximately 3 minutes and sealing by immersion forapproximately 45 minutes. The workpiece was then dried with air, inparticular at a low temperature, if good properties of adhesiveness wererequired for the oxide.

[0021] The properties of this oxide are indicated in Table I.

EXAMPLE 2 Anodizing a 7000-Series Aluminium Alloy withSulphuric-Tartaric Acid

[0022] A workpiece (of a 7000-series aluminium alloy having dimensionsof 150×100×1 mm) was first subjected in a conventional manner tocleaning and deoxidizing treatments, similar to those described withreference to Example 1.

[0023] The workpiece was then used as the anode of an electrochemicalcell, in which the cathode was constituted by corrosion-resistant steelof the type AISI 321 and having a surface area equal to or greater thanthat to be oxidized and in the presence of an aqueous acid solutioncontaining 45 g sulphuric acid and 80 g L(+)-tartaric acid per liter ofsolution and was maintained at a temperature of between 30 and 35° C.The workpiece was kept completely immersed in the acid solution.

[0024] The electrical voltage was increased from an initial value of 0 Vto a value of 15 V at a rate in the order of 3 V/min. The final voltagevalue was maintained for 25 minutes, producing on the workpiece, uponcompletion of the anodizing treatment, an oxidized coating having athickness of approximately 5 μm.

[0025] The anodized workpiece was then subjected to rinsing and sealingtreatments, which are conventional per se, similar to those describedwith reference to Example 1.

[0026] The properties of the oxide coating obtained are indicated inTable I.

EXAMPLE 3 Anodizing Plated or Pure Aluminium with Sulphuric-TartaricAcid

[0027] A workpiece (of a plated aluminium alloy or pure aluminium havingdimensions of 150×100×1 mm) was first subjected, in a conventionalmanner, to cleaning and deoxidizing treatments, similar to thosedescribed with reference to Example 1.

[0028] The workpiece was then used as the anode of an electrochemicalcell, in which the cathode was constituted by corrosion-resistant steelof the type AISI 321 and having a surface area equal to or greater thanthat to be oxidized and in the presence of an aqueous acid solutioncontaining 45 g sulphuric acid and 80 g L(+)-tartaric acid per liter ofsolution and was maintained at a temperature of between 30 and 35° C.The workpiece was kept completely immersed in the acid solution.

[0029] The electrical voltage was increased from an initial value of 0 Vto a value of 15 V at a rate in the order of 3 V/min. The final voltagevalue was maintained for 25 minutes, producing on the workpiece, uponcompletion of the anodizing treatment, an oxidized coating having athickness of approximately 5 μm.

[0030] The anodized workpiece was then subjected to rinsing and sealingtreatments, which are conventional per se, similar to those describedwith reference to Example 1.

[0031] The properties of the oxide coating obtained are indicated inTable I.

EXAMPLE 4 (FOR COMPARATIVE PURPOSES) Anodizing with Chromic Acid

[0032] A workpiece (of a plated aluminium alloy or a pure aluminiumhaving dimensions of 150×100×1 mm) was first subjected in a conventionalmanner to cleaning and deoxidizing treatments, similar to thosedescribed with reference to Example 1.

[0033] The workpiece was then used as the anode of an electrochemicalcell, in which the cathode was constituted by corrosion-resistant steelof the type AISI 321 and having a surface area equal to or greater thanthat to be oxidized and in the presence of an aqueous acid solutioncontaining 65 g chromic acid and was maintained at a temperature ofbetween 35 and 40° C. The workpiece was kept completely immersed in theacid solution.

[0034] The electrical voltage was increased from an initial value of 0 Vto a value of 40 V at a rate in the order of 5 V/min. The final voltagevalue was maintained for 45 minutes, producing on the workpiece, uponcompletion of the anodizing treatment, an oxidized coating having athickness of approximately 3 μm.

[0035] The anodized workpiece was then subjected to rinsing and sealingtreatments, which are conventional per se, similar to those describedwith reference to Example 1.

[0036] The properties of the oxide coating obtained are indicated inTable I. TABLE I Prior art Chromic acid Prior art INVENTION PropertyTest method (Example 4) Sulphuric acid (Examples 1, 2, 3) Weight ofoxide ASTM B 137 >32 mg/dm² >32 mg/dm² >40 mg/dm² Corrosion resistanceTests in saline mist ac- No corrosion points after No corrosion pointsafter No corrosion points after cording to Iso 7253 exposure for 500hours exposure for 500 hours exposure for 750 hours Impact testaccording to No detachment of paint or ASTM D 2794 only slight crackingof paint NOT APPLICABLE No detachment of paint Paint adhesion Adhesionwithout condi- tioning according to ISO 100% adhesion 90% adhesion 100%adhesion 2409 Adhesion after condition- ing in water for 14 days 100%adhesion 80% adhesion 100% adhesion according to ISO 2409 Influence ofoxide on Fatigue tests on cylindrical Reduction in fatigue cyclesReduction in fatigue cy- fatigue life of base test pieces at frequencyof not greater than 25% Unsatisfactory cles not greater than 15% metal10-140 Hz, ratio R:0.1

[0037] A comparison of the results of the tests carried out on theworkpieces of the Examples according to the invention and the Examplefor comparative purposes establishes that the process according to theinvention produces oxides having properties which are superior to thoseof the oxides obtained with conventional processes. Furthermore, theprocess according to the invention offers environmental advantagesassociated with the different types of product used, and has a durationwhich is substantially shorter.

[0038] Of course, without altering the principle of the invention, thedetails of implementation and the embodiments can vary extensively inrelation to the description, which is given purely by way of example,without in any way departing from the scope thereof. In particular, theaqueous acid solution could contain, in addition to sulphuric acid andtartaric acid, any other components or combinations of components whichare compatible with those substances.

What is claimed is:
 1. Process for anodizing a workpiece of aluminium oraluminium alloys, which provides for the use of the workpiece as theanode of an electrolytic cell in the presence of an aqueous acidsolution, wherein the aqueous acid solution contains from 10 to 200 gsulphuric acid and from 5 to 200 g L(+)-tartaric acid.
 2. Processaccording to claim 1, wherein the aqueous acid solution contains from 20to 80 g sulphuric acid and from 30 to 120 g L(+)-tartaric acid per literof solution.
 3. Process according to either of the preceding claims,wherein the solution is maintained, during the anodizing treatment, at atemperature of between ambient temperature and 120° C., and preferablybetween 25 and 35° C.
 4. Process according to claim 1, wherein theelectrolytic cell is subjected to a voltage of between 1 and 120 V, andpreferably of between 10 and 30 V.
 5. Process according to claim 1,wherein it has a duration of between 5 and 120 minutes, and preferablyof between 5 and 30 minutes.
 6. Process according to claim 1, wherein itis preceded by a cleaning, deoxidizing and rinsing treatment for theworkpiece to be anodized.
 7. Process according to claim 1, wherein it isfollowed by a rinsing and sealing treatment for the anodized workpiece.